1. Field of the Invention
The present invention relates to an optical pickup device or optical head device and particularly to an optical system of an optical pickup device in an optical recording and reproducing apparatus.
2. Description of the Related Art
There have been known as optical information recording mediums, various kinds of optical discs, called Laser Disc (LD), Compact Disc (CD) and Digital Video Disc (DVD). Such optical discs are standardized respectively in different specifications such as a base thickness of the substrate and the like. An optical disc having a multi-layered structure provided in a DVD specification comprises many laminated recording layers having effective thicknesses different from each other. Moreover, the most suitable numerical aperture (NA) of a reading objective lens is different among the different optical discs.
For example, a compatible player capable of reading out recording information from both the CD and DVD is required in the market. There are the following differences between optical systems for reading out the recording information on the CD and the DVD.
(1) The difference of numerical apertures NA: the numerical aperture for the CD is 0.37 and for the DVD the numerical aperture is 0.6.
(2) The difference of base thicknesses from a recording surface (reflection surface) to an outer surface of the optical disc: the base thickness for the CD is 1.2 mm and for the DVD the base thickness is 0.6 mm.
Consequently, these differences must be canceled to achieve the optical pickup device for the CD/DVD compatible player.
To achieve such compatible players, it is often thought that an objective lens having two focal points should advantageously be employed.
Specifically, an objective lens having a single focal point has a numerical aperture most suitable for one optical disc, the numerical aperture being improper for the other optical disc. For the other optical disc, such objective lens produces an aberration such as a spherical aberration.
As an example, a two-focal-point pickup device using a hologram lens has been disclosed in Japanese Patent Application Kokai NO. 7-98431/1995. This two-focal-point pickup device includes a complex objective lens which is composed of a convex objective lens 1 and a hologram lens 2 for reading the different optical discs, as shown in FIGS. 1A and 1B. In the hologram 2, a plurality of diffraction grooves 3 are formed over the region corresponding to the numerical aperture for the CD. A light beam is diffracted by these diffraction grooves 3, so that it is mainly divided into a zero-order diffraction light and a first-order diffraction light. Thus, respective incident angles of light onto the objective lens 1 are made different, whereby each focal point is formed on corresponding one of recording surfaces of the optical discs. At this time, the light beam is allowed to transmit as it is through the region where no diffraction groove is formed, without being diffracted, and this transmitted light is collected by the objective lens 1 together with the zero-order diffraction light. As a result, there will be different in the numerical aperture between the transmitted light and the zero-order diffraction light, and the first-order diffraction light. As shown in FIG. 1A, only the first-order diffraction light diffracted by the diffraction grooves 3 is employed for reading the CD 4 of the small numerical aperture. As shown in FIG. 1B, the transmitted light and the zero-order diffraction are employed for reading the DVD 5 of the large numerical aperture. As seen from these figures, the hologram lens functions as a concave lens for the first-order diffraction light beam in this complex objective lens for reading the optical disc. This is because a plurality of diffraction grooves 3 or ring-shaped concave and convex portions are evenly formed on a transparent flat plate over the region corresponding to the numerical aperture for the CD.
On the other hand, the semiconductor laser emitting a divergent light beam in the CD/DVD compatible player is generally used as a light source in the optical pickup device. The far field pattern of the light beam has an elliptic cross sectional intensity having a major longitudinal axis extending in the direction perpendicular to the junction interface of the semiconductor laser while radiating light with a radiating angle .theta. in relation of .theta..perp.&gt;.theta..parallel.. In other words, the light beam from the semiconductor laser has a width of the vertical traversal mode smaller than the horizontal traversal mode. In addition, the signal reproduction from DVD has a tendency of the increase of cross-talk caused by the adjacent tracks in the reproduced signal in comparison with the conventional CD, since the track-pitch of the recording pits of the DVD is narrower than that of the CD. In the view of this tendency, the semiconductor laser for DVD is generally disposed in the optical pickup device in such a manner that its horizontal traversal mode corresponds to the track extending direction (also referred to as a tangential direction) of the optical disc, and its vertical traversal mode corresponds to the radial direction of the optical disc. Therefore, as shown in FIG. 2, the light spot 12 for DVD caused by the light beam converged on a series of the pits 11 of the recording surface 10 of the optical disc becomes an ellipse extending in the tangential direction.
Since only one light beam emitted from a single semiconductor laser is converged by the common complex objective lens to be commonly used for the reproduction of CD and DVD in the two-focal-point pickup device, a light spot 13 for reading CD in the first-order diffraction light has an elliptic shape extending in the tangential direction as well as a light spot 12 for reading DVD as shown in FIGS. 2 and 3.
In order to precisely reproduce data from the DVD of a high recording density in the CD/DVD compatible player, it is necessary to employ the semiconductor laser emitting light with a short wavelength from 630 to 680 nm less than the 780 nm wavelength of the conventional. For this, the numerical aperture of the objective lens for reading the CD used in the CD/DVD compatible player will be set a value rather smaller than that of the conventional CD player. In fact, the numerical aperture for CD is set a little value larger than a numerical aperture decided in proportion to the used wavelength in the two-focal-point pickup device to avoid the reduction of light intensity depending on the first-order diffraction light and the small numerical aperture. Therefore, a light spot 13 is formed on the recording surface 10 of the CD by the two-focal-point pickup device in such a manner that the diameter of beam west of the formed light beam 13 is reduced less than the conventional light spot size used for reading the CD as shown in FIG. 3.
In this case, if the first-order diffraction light is converged as the light spot 13 on a long pit 11 of a series of pits as corresponding to binary digit data "1001110" formed on the CD as shown in FIG. 4, then a glitch or bump A occasionally occurs in the signal intensity curve of the output from the optical pickup device, resulting in a problem that a good signal is not obtained.